Device for laser-ultrasonic detection of flip chip attachment defects

ABSTRACT

A device detects underfill voids and solder ball defects via laser generation and laser detection of an ultrasonic wave at the top surface of flip chips. High resolution is provided by using small laser spot sizes and closely-spaced laser beams of wavelengths that are absorbed near the surface of the semiconductor. Improved spatial resolution and rejection of unwanted scattered waves can be attained by limiting the time frame of the ultrasonic waveform to the time required for the first longitudinal wave reflection from the bottom of the flip chip. The laser beam spacing can be reduced by using probe and detection beams of different wavelengths. Resolution of less than 100 μm features was demonstrated for silicon flip chips.

RELATED APPLICATION

The present application is a Divisional Application claiming priority toU.S. patent application Ser. No. 10/903,557 filed 29 Sep. 2004. Bothapplications have the same inventors and the same assignees.

U.S. GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.DTRA01-03-C-0030 awarded by the Missile Defense Agency. The Governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to electronics assembly andis more specifically concerned with quality control of flip chipattachment.

2. Description of the Related Art

Modern electronic assemblies generally contain integrated circuits anddigital memory incorporated in semiconductor chips. In early approaches,chips were mounted and contacted electrically via wire bonds runningfrom metallic contact pads around the periphery of the top side of thechip to mating pads on a polymer-based or ceramic chip carrier orsubstrate. The carrier and chip were typically contained in a packagethat protected the chip, and had solderable leads for attachment to acircuit board. Later approaches involved wire bonding the chip directlyto pads on a circuit board so as to eliminate the package and reduce thefootprint of the device. As electronics miniaturization progressed,chips become so small and densely packed with circuitry and circuitelements that even fine-pitched peripheral contacts could no longerprovide the needed number of input/output (I/O) connections.Furthermore, wire bond connections introduced appreciable inductiveimpedance, which limited the device switching speed. These issues wereaddressed by distributing the electrical contact pads in an area arrayover one side of the chip, and making electrical contacts via shortsolder connections.

In the flip chip approach, solder balls are first attached to an arrayof contact pads distributed over the “top” or active surface of the chipto form a solder ball array. The chip is then flipped upside down andpositioned so that the solder balls are aligned with mating contact padson the substrate, which is typically a polymer-based or ceramic circuitboard. The solder interconnections are made by reflow soldering. Theempty space between the bottom of the flip chip and the substrate isgenerally filled with an epoxy underfill material to reducethermally-induced strain that could cause fatigue cracking of the solderinterconnections. For some applications, underfill is also required forheat removal and to provide resistance to acceleration-induced strain.The underfill material, which is typically a proprietary epoxyformulation, is generally injected as a liquid so that air bubblessometimes produce underfill voids that can cause localizedheating/stresses, leading to premature failure. The solder connectionsmay also have inadequate electrical or mechanical properties due tovoids, disbonds or insufficient solder ball volume. A means forcontrolling such underfill and solder defects is needed to improveprocess yields and to ensure high reliability for critical applications.

The only practical prior art technique for detecting underfill voids isscanning acoustic microscopy, which requires that the part be immersedin water (or another fluid). This technique can be applied only afterthe epoxy underfill has cured, which precludes reworking the part sincethe cured epoxy cannot be removed without damaging the chip. Waterimmersion of the part is also incompatible with in-line inspection sothat only a small fraction of the parts can be tested, which providesonly statistical process control. A method enabling in-line detection offlip chip attachment defects would provide great benefits in terms ofpart reliability and process yields.

U.S. Pat. No. 5,585,921 to Pepper et al. describes a laser-ultrasonicsystem applied to on-line detection of welding defects. In this case,one laser was used to generate an array of acoustic waves within theworkpiece and a second laser, coupled with an interferometer, was usedto detect vibrations of the workpiece surface produced by thelaser-generated acoustic waves. The magnitude of the measured acousticwaves was increased via reflections from incompletely formed welds. Thearray of acoustic waves was generated and detected via full or partialconcentric ring-shaped laser beams designed so that the component wavesarrived at the detection site at the same time and were reflectedin-phase. This focused the ultrasonic energy on the weld area, enhancingthe signal strength and reducing the effects of speckle reflections fromrough weld surfaces. Nonetheless, the width of the focus area even withthe acoustic wave array was about 1 mm, which is an order of magnitudelarger than the resolution needed for detection of flip chip attachmentflaws (<100 μm). Such prior art implies that the laser-ultrasonicapproach may not be applicable to the flip chip application.

Pepper et al. (Rev. Prog. Quant. NDE, Vol. 17, Plenum Press, New York,1998) describe void detection in a flip chip package via alaser-ultrasonic technique involving ultrasonic generation on thepolymer-based substrate (FR4 material) and detection on the oppositeside of the package, i.e., at the top surface of the flip chip. Thegeneration laser employed had a relatively large spot size (˜0.5 mmdiameter), which enhanced ultrasonic wave generation with minimallaser-induced damage to the substrate but severely limited theattainable resolution. Nonetheless, the laser power used exceeded theablation threshold of the FR4 substrate material, rendering thetechnique partially destructive. Since this prior art approach providedlimited resolution, relatively large laser scanning steps (100 μm) wereused.

SUMMARY OF THE INVENTION

The present invention provides a laser-ultrasonic device that is usefulfor detection of defects in the underfill and solder ball attachmentsbetween a flip chip and a substrate. The device of the inventioncomprises a generation laser, a detection laser, an interferometer andan analyzer, and may further comprise a translation stage. Thegeneration laser of the device is configured to provide a generationlaser beam of small diameter directed to a predetermined generation spoton the top surface of the flip chip so as to generate a probe acousticwave in the flip chip material. The detection laser of the device isconfigured to provide a detection laser beam of small diameter thatimpinges the top surface of the flip chip at a detection spot, having apredetermined spatial relationship to the generation spot. Defects inthe underfill and solder ball attachments between the flip chip and thesubstrate are detected via the temporal displacement of the top surfaceof the flip chip at the detection spot produced by an acoustic wavereflected from the bottom surface of the flip chip, whose intensity ismodulated by such defects. This surface temporal displacement isdetected using the interferometer of the device of the invention, whichis configured to detect a portion of the detection laser beam reflectedfrom the flip chip surface. The output from the interferometer istypically an acoustic waveform (surface displacement magnitude vs.time). The analyzer of the device of the invention compares acousticwaveforms for a plurality of detection spots to detect defects in theunderfill and solder ball attachments. Preferably, the flip chip ismounted on a translation stage, which precisely positions the probedarea on the flip chip surface relative to the laser beams, and providesraster scanning.

The acoustic bandwidth may be maximized by using a generation laser of awavelength that is strongly absorbed by the flip chip semiconductor sothat the laser light is absorbed in a thin region near the top surfaceof the flip chip. Alternatively, the wavelength of the generation laserlight may be selected so as to distribute the light absorption over agreater volume of the semiconductor to enhance the directivity andintensity of the acoustic wave while avoiding ablation damage to theflip chip. The detection laser beam preferably has a wavelength that isstrongly absorbed by the flip chip semiconductor so that it efficientlysenses the motion of the flip chip top surface (laser entrance face).

In a preferred embodiment, high sensitivity and resolution are attainedvia use of very small laser spot diameters and a close spacing betweenthe generation laser spot and the detection laser spot. In this case,only those ultrasonic waves that travel in a narrow angular range,defining a small probe area at the bottom surface of the chip, aredetected so that very small voids and defects can be resolved. For thethin silicon chips normally employed, features smaller than thewavelength of the generated ultrasonic wave can be resolved. Thegeneration laser spot and the detection laser spot may overlap if lasersof two different wavelengths are used.

The detection of the reflected ultrasonic wave may be time gated so thatwaves arriving at longer or shorter times compared to those for apredetermined time range are not detected. Thus, reflected ultrasonicwaves that must travel a shorter or longer distance to arrive at thedetection spot (compared to those from the defined probe area) arerejected. This approach provides further spatial selectivity for thewaves reflected from the defect location being probed, and enablesrejection of non-specular reflections from areas outside the probe areafor flip chips with scattering features at the bottom surface.

The device of the invention may be used to provide an image of underfilland solder ball defects. In this case, measurements of acousticwaveforms (surface displacement magnitude vs. time) are made atregularly spaced locations along the flip chip surface by x-y rasterscanning of the laser beams or the flip chip, while a predeterminedspatial relationship is maintained between the generation laser spot andthe detection laser spot. Enhanced resolution and sensitivity may beprovided via more sophisticated signal processing.

In a preferred embodiment, a single waveform corresponding to adefect-free location is chosen as a reference, and the overall amplitudeof each waveform is normalized to the amplitude of the referencewaveform. The time frame considered for the normalized waveforms ispreferably gated to the arrival time of the first longitudinal wavereflection from the probe area at the bottom surface of the flip chip. Acomputer program is preferably used to calculate the Mean Square Error(MSE) between the reference waveform and each of the other waveforms inthe raster scan. A plot of MSE intensity versus x-y location provides animage of the bottom side of the flip chip.

The present invention provides significant advantages compared to priorart devices and methods. A key advantage is that the laser-ultrasonicdevice and method of the invention can be used for in-line detection offlip chip attachment defects, enabling 100% parts inspection andreal-time process control. Since fluid immersion is not required,laser-ultrasonic inspection can be applied before the epoxy underfillhas cured so that defective parts can be reworked when necessary. Thepresent invention also provides high defect sensitivity and resolution.For the thin semiconductor layers typically employed for flip chips,attachment defects smaller than the wavelength of the laser-generatedultrasonic wave can be detected. The present invention may be used toimprove the reliability of flip chip parts and increase the yield offlip chip assembly processes.

Further features and advantages of the invention will be apparent tothose skilled in the art from the following detailed description, takentogether with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of the laser-ultrasonic device of theinvention and a schematic cross-section illustrating use of the devicefor detection of an attachment defect for a flip chip having asubstantially smooth bottom surface for which reflection of ultrasonicwaves is specular.

FIG. 2 depicts a block diagram of the laser-ultrasonic device of theinvention and a schematic cross-section illustrating use of the devicefor detection of an attachment defect for a flip chip having scatteringfeatures at the bottom surface so that the reflection of ultrasonicwaves is non-specular.

FIG. 3 shows an image of a chip scale package solder bump array withmissing solder balls generated using the laser-ultrasonic device of thepresent invention.

FIG. 4 shows an image of a flip chip underfill void generated using thelaser-ultrasonic device of the present invention.

These figures are not drawn to scale. Some features have been enlargedrelative to other features for ease of depiction.

DETAILED DESCRIPTION OF THE INVENTION

In this document, the term “flip chip” is used in the broadest sense andincludes any semiconductor chip having an area array of electricalcontacts on one surface. Thus, the term “flip chip” encompasses chips inchip scale packages, which may be contacted to a circuit board via aflex circuit and solder balls or bumps. The words “acoustic” and“ultrasonic” are used interchangeably.

The present invention provides a laser-ultrasonic device that is usefulfor detection of defects in the underfill and solder ball attachmentsbetween a flip chip and a substrate. A method for using the device forthis purpose is described in U.S. patent application Ser. No. 10/903,557(filed 29 Sep. 2004), which is the parent to the present divisionalapplication. A flip chip includes an area array of solder balls betweencontact pads on the flip chip and contact pads on the substrate. Thesolder ball attachment is typically made by reflow soldering. Thesubstrate may comprise any polymer-based material, FR4 material, forexample, or any ceramic material, alumina, for example.

The present invention may be applied to detection of attachment defectsfor flip chips comprising any semiconductor material, including silicon,germanium, gallium arsenide, indium phosphide, and silicon carbide. Thebottom of the flip chip may have a substantially smooth surface forwhich reflection of acoustic waves is specular, or may includescattering features so that reflection of acoustic waves from the bottomsurface of the flip chip is non-specular. Attachment defects that may bedetected include underfill voids, underfill disbonds, missing solderballs, disbanded solder balls, and solder balls with insufficient soldermass. The flip chip surface may include circuitry lines and devices,which are usually smaller than the desired resolution and are notdetected.

The device of the present invention for detecting a defect in anattachment between a flip chip and a substrate, comprises: (1) ageneration laser providing a generation laser beam that impinges the topsurface of the flip chip at a predetermined generation spot andgenerates an acoustic wave within the flip chip; (2) a detection laserproviding a detection laser beam that impinges the top surface of theflip chip at a detection spot having a predetermined spatialrelationship to the predetermined generation spot; (3) an interferometerproviding an acoustic waveform of the temporal displacement of the topsurface of the flip chip at the detection spot based on the magnitude ofa portion of the detection laser beam reflected from the top surface ofthe flip chip; and (4) an analyzer that compares the acoustic waveformsfor a plurality of predetermined generation and detection spots on thetop surface of the flip chip to detect the defect in the attachmentbetween the flip chip and the substrate.

In practice, the generation laser beam generates a probe acoustic wavewithin the flip chip and the interferometer detects a reflected acousticwaveform for a plurality of predetermined generation and detection spotson the top surface of the flip chip, and the analyzer compares thereflected acoustic waveforms detected for at least two predetermineddetection spots to detect the defect in the attachment between the flipchip and the substrate. The device of the invention may further comprisea translation stage for x-y raster scanning the flip chip top surfacerelative to the generation and detection laser spots while acousticwaveforms are measured at predetermined locations along the flip chiptop surface to provide an image of attachment defects at the bottomsurface of the flip chip.

FIG. 1 depicts a block diagram of the device of the invention and aschematic cross-section illustrating use of the device for detection ofan attachment defect 107 for a flip chip 102 having a substantiallysmooth bottom surface 104 for which reflection of ultrasonic waves isspecular. The device of the invention comprises a generation laser 121(which provides generation laser beam 100), a detection laser 122 (whichprovides detection laser beam 110), an interferometer 124, an analyzer125, and, optionally, a translation stage 130. Generation laser beam 100incident at a generation spot on top surface 101 of flip chip 102generates an ultrasonic wave 103 (in flip chip 102) that diverges fromthe generation spot and is specularly reflected from smooth bottomsurface 104 of flip chip 102. Bottom surface 104 is in contact withunderfill 105 between flip chip 102 and substrate 106. Ultrasonic wave Dreflected from bottom flip chip surface 104 within a small probe area(not shown) impinges a detection spot on top surface 101, defined bydetection laser beam 110, and causes a localized temporal displacementof surface 101. A reflected beam 123, which is a portion of detectionlaser beam 123 that is reflected from surface 101, is analyzed via aninterferometer to generate an acoustic waveform for ultrasonic wave D.When, as in FIG. 1, the probe area fully or partially overlaps a defectin underfill 105, such as void 107, the intensity of the acousticwaveform is typically enhanced. Void 107 is detected by comparing theacoustic waveforms for a plurality of generation and detection spots ontop surface 101 of flip chip 102. Since the angle of reflectance frombottom surface 104 for an ultrasonic wave generated by generation laserbeam 100 equals the angle of incidence, ultrasonic waves A, B, C and Ereflected from areas of bottom surface 104 outside the probe area do notimpinge the detection spot and are not detected. This enhances thesignal to noise ratio so that high spatial sensitivity can be attained.Thus, the device of the present invention permits interrogation of asmall probe area at the bottom surface of a flip chip via ultrasonicwaves traveling in a narrow spatial and angular range defined by theflip chip thickness and the spacing and spot diameters of the generationand detection laser beams.

For prior art laser-ultrasonic detection methods, attainable resolutionis limited by divergence of the generated ultrasonic wave, and by therelatively large probe areas typically involved. We have discovered,however, that very small flip chip attachment defects can be detected bygenerating and detecting the ultrasonic wave on the top surface of theflip chip using closely-spaced laser beams of small spot diameters. Thiscontrasts with the prior art approach for laser-ultrasonic inspection offlip chips via generation and detection on opposite sides of the flipchip (D. M. Pepper, G. J. Dunning, M. P. Chiao, T. R. O'Meara and P. V.Mitchell, Rev. Prog. Quant. NDE, Vol. 17, Plenum Press, New York, 1998).

With generation and detection on top flip chip surface 101 (FIG. 1),according to the present invention, divergence of ultrasonic wave 103 isminimized since the generated and reflected waves must traverse only thethickness of flip chip 102. Since the thickness of flip chip 102 istypically very small (<1 mm), practically the full intensity oflaser-generated ultrasonic wave 103 is delivered to bottom surface 104.This maximizes the intensity of reflected ultrasonic wave D and thestrength of the signal detected via detection laser beam 110. In thiscase, attainable resolution is limited by the spacing and spot sizes ofthe laser beams and the signal processing efficiency, and not by thewavelength of the ultrasonic wave. For closely-spaced laser beams ofsmall spot diameter, we have shown that the dimensions of the probe areafor the reflected ultrasonic beam can be much smaller than thewavelength of the highest frequency ultrasonic wave.

The method of the invention for detecting a defect in an attachmentbetween a flip chip and a substrate comprises the steps of: (1)generating a probe acoustic wave within the flip chip by directing ageneration laser beam to a predetermined generation spot on the topsurface of the flip chip; (2) detecting a reflected acoustic waveformvia an interferometer and a detection laser beam that impinges the topsurface of the flip chip at a detection spot having a predeterminedspatial relationship to the generation spot; (3) repeating steps (1) and(2) for a plurality of predetermined generation and detection spots onthe top surface of the flip chip; and (4) comparing the reflectedacoustic waveforms detected for at least two predetermined detectionspots to detect the defect in the attachment between the flip chip andthe substrate.

The generation laser beam preferably comprises a single pulse so as toprovide maximum ultrasonic wave amplitude without substantial damage tothe flip chip. Multiple generation laser pulses could be used. Thegeneration laser pulse energy is preferably below the ablationthreshold, which depends on the semiconductor material and thewavelength of the laser light. In the thermoelastic regime below theablation threshold, the laser beam generates two types of acoustic wavesthat may be used to interrogate the flip chip bottom surface.Compressional waves, which are relatively weak and travel primarilyalong the normal to the flip chip surface, can only be detected when thegeneration and detection laser beams are very close together, preferablyoverlapped. The pulse width of compressional waves is limited only bythe temporal width of the laser pulse. Shear waves, which are relativelystrong, tend to travel at angles to the surface normal. The pulse widthof shear waves is also limited only by the temporal width of the laserpulse, provided that the generation laser spot size is sufficientlysmall.

The wavelength of the generation laser beam is preferably predeterminedsuch that the generation laser light is absorbed in a relatively thinregion near the top surface of the flip chip. When the photon energy ofthe generation laser exceeds the bandgap energy of the semiconductormaterial comprising the flip chip, the laser light is absorbed in a verythin region near the flip chip top surface, which maximizes the acousticbandwidth. Especially for a semiconductor material that has an indirectbandgap, such as silicon, a sub-bandgap laser photon energy may beemployed to distribute the light absorption over a greater volume in thebulk of the semiconductor so as to enhance the directivity and intensityof the acoustic wave while avoiding ablation damage to the flip chip.The photon energy of the detection laser is preferably predetermined toexceed the bandgap energy of the semiconductor so that light penetrationis minimized, providing good sensitivity to temporal displacement of thetop surface. Note that even a strongly absorbing material has anintrinsic Fresnel reflectivity, enabling laser-interferometer detectionof the top surface displacement.

The sensitivity and resolution provided by the invention are enhanced byuse of small laser spot sizes and a close spacing between the generationand detection laser spots. The spot diameter for both the generation anddetection laser beams should be as small as practical, preferably 100 μmor less. The center-to-center spacing between the generation anddetection laser spots should be less than 300 μm. If the generation anddetection laser beams have different wavelengths, the generation anddetection spots may be overlapped to increase signal strength andimprove the signal-to-noise ratio.

The ultrasonic wave reflected from the bottom surface of the chip, whoseintensity is modulated by a void in the underfill or a defect in asolder ball, is detected via the temporal displacement of the topsurface of the flip chip produced by the reflected acoustic wave. Thissurface displacement is measured using an interferometer and a detectionlaser beam of small diameter that impinges the top surface of the flipchip at a detection spot. Suitable interferometer-laser vibrationdetection equipment and methods are known in the art. A preferreddetection scheme involves the use of two-wave mixing in aphotorefractive crystal. The crystal acts as an adaptive beam combiner,allowing interrogation of rough surfaces and avoiding the need for anypath-length stabilization in the interferometer.

For silicon flip chips, 532-nm light, which may be provided by afrequency-doubled Nd:YAG laser, may be used for both generation anddetection. Such above-bandgap light is absorbed within about 0.1 μm intothe silicon surface, providing high acoustic bandwidth. Alternatively,1064 nm light, which may be provided by a fundamental Nd:YAG laser, maybe used for acoustic wave generation in silicon. Such near-bandgap lightpenetrates to a depth of about 200 μm into the silicon bulk. Sinceabsorption of 1064-nm light is distributed over a relatively largesilicon volume (compared to 532-nm light), a laser pulse of higherenergy may be used to enhance ultrasonic wave generation withoutexceeding the ablation threshold. The buried nature of 1064-nmabsorption in silicon and the resulting mechanical clamping ensure thatstrong compressional ultrasonic waves are produced, with a peakdirectivity along the normal to the surface. The distributed nature ofthe 1064-nm absorption, however, leads to time broadening of thepropagating ultrasonic pulse, thereby reducing its bandwidth. Thereduced bandwidth in turn leads to acoustic waves of longer wavelength,which can still be used to detect small flip chip attachment defects.Overlap of the generation and detection beams to enhance resolution forsilicon chips may be provided by using 532-nm light for generation and515-nm light for detection.

Within the scope of the present invention, signal-to-noise ratio may beenhanced by utilizing a predetermined time frame for the waveformscorresponding to the arrival time of the first longitudinal wavereflected from the bottom surface of the chip. This time gating approacheffectively rejects contributions from ultrasonic waves reflectedoutside the probe area, which must traverse a greater or smallerdistance and arrive at the detection spot at times outside thepredetermined time frame.

FIG. 2 depicts a block diagram of the laser-ultrasonic device of theinvention and a schematic cross-section illustrating use of the devicefor detection of an attachment defect for a flip chip having scatteringfeatures at the bottom surface, which may be associated with integratedcircuit conductor lines or electronic devices, for example. The deviceof the invention comprises a generation laser 221 (which providesgeneration laser beam 200), a detection laser 222 (which providesdetection laser beam 210), an interferometer 224, an analyzer 225, and,optionally, a translation stage 230. Generation laser beam 200 incidentat a generation spot on top surface 201 of flip chip 202 generates anultrasonic wave 203 (in flip chip 202) that is reflected or scatteredfrom bottom surface 204, which is in contact with underfill 205 betweenflip chip 202 and substrate 206. Ultrasonic wave G reflected from bottomflip chip surface 204 within a small probe area (not shown) impinges adetection spot on top surface 201, defined by detection laser beam 210.Since scattering of acoustic waves from scattering features at bottomsurface 204 occurs over a range of angles, ultrasonic waves F and Hreflected from bottom surface 204 outside the probe area also impingethe detection spot on top surface 201 (defined by laser beam 210) andare detected as false signals. However, the distance to the detectionspot for waves F and H are longer and shorter, respectively, compared tothe signal wave G. Thus, the false signals from waves F and H can beeliminated by time gating with respect to the signal wave G, i.e.,rejecting waves arriving at the detection spot at shorter or longertimes compared to signal wave G. Time gating also tends to enhance thespatial resolution for defect detection.

In a preferred embodiment, a single waveform corresponding to adefect-free location is chosen as a reference, and the overall amplitudeof each waveform is normalized to the amplitude of the referencewaveform. The normalized waveforms are preferably time gated at thearrival time of the first longitudinal wave reflected from the bottomsurface of the chip. A computer program is preferably used to calculatethe Mean Square Error (MSE) between the reference waveform and each ofthe other waveforms in the raster scan. The MSE values are used as ameasure of the defect scattering level. A plot of MSE intensity versusx-y location provides an image of the bottom side of the flip chip.

DESCRIPTION OF A PREFERRED EMBODIMENT

The efficacy of the present invention was demonstrated by generating alaser-ultrasonic images of an underfill void and missing solder ballsfor silicon flip chips (<600 μm thick) attached to an FR4 substrate. Asingle waveform corresponding to a defect-free location was chosen as areference, and the overall amplitude of each waveform was normalized tothe amplitude of the reference waveform. The time frame considered forthe normalized waveforms was then gated to the arrival time of the firstlongitudinal wave reflection from the probe area at the bottom surfaceof the flip chip. A computer program was used to calculate the MeanSquare Error (MSE) between the reference waveform and each of the otherwaveforms in the raster scan. A plot of MSE intensity versus x-ylocation provided an image of the bottom side of the flip chip.

FIG. 3 shows an image of a chip scale package solder ball arraygenerated by the laser-ultrasonic device of the present invention. Thesolder balls were 435 μm in diameter and the pitch of the array was 750μm. Ultrasonic wave generation was provided by a 532-nm doubled Nd:YAGlaser (0.64 mJ/pulse) with a 100 μm spot size. The detection laser had awavelength of 515 nm, a spot size of 100 μm, and a power output of 50mW. Acoustic waveforms of surface displacement magnitude vs. time weremeasured at 60-μm steps along the flip chip surface by x-y rasterscanning. The generation and detection laser spots were overlapped onthe top surface. The scan area was 3 mm×3 mm in size. Two missing solderballs are clearly evident.

FIG. 4 shows an image of a flip chip underfill void generated by thelaser-ultrasonic device of the present invention. The solder balls were135 μm in diameter and the pitch of the array was 254 μm. Ultrasonicwave generation was provided by a 1064-nm Nd:YAG laser (4.37 mJ/pulse)with a 100 μm spot size. The detection laser had a wavelength of 532 nm,a spot size of 20 μm, and a power of 50 mW. Acoustic waveforms ofsurface displacement magnitude vs. time were measured at 80-μm stepsalong the flip chip surface by x-y raster scanning. The generation anddetection laser spots were overlapped on the top surface. The scan areawas 4.4 mm×4.4 mm in size. The frequencies of the generated ultrasonicwaves ranged from 1 to 10 MHz, corresponding to 8 to 0.8 mm wavelengths.The resolution attained was less than 100 μm, which is nearly an orderof magnitude smaller than the shortest ultrasonic wavelength. Aresolution of about 20 μm should be attainable by use of optimized timegating and very small generation and detection laser spot sizes.

The preferred embodiments of this invention have been illustrated anddescribed above. Modifications and additional embodiments, however, willundoubtedly be apparent to those skilled in the art. Furthermore,equivalent elements may be substituted for those illustrated anddescribed herein, parts or connections might be reversed or otherwiseinterchanged, and certain features of the invention may be utilizedindependently of other features. Consequently, the exemplary embodimentsshould be considered illustrative, rather than inclusive, while theappended claims are more indicative of the full scope of the invention.

1. A device for detecting a defect in an attachment between a flip chipand a substrate, comprising: a generation laser providing a generationlaser beam that impinges the top surface of the flip chip at apredetermined generation spot and generates an acoustic wave within theflip chip; a detection laser providing a detection laser beam thatimpinges the top surface of the flip chip at a detection spot having apredetermined spatial relationship to the predetermined generation spot;an interferometer providing an acoustic waveform of the temporaldisplacement of the top surface of the flip chip at the detection spotbased on the magnitude of a portion of the detection laser beamreflected from the top surface of the flip chip; and an analyzer thatcompares the acoustic waveforms for a plurality of predeterminedgeneration and detection spots on the top surface of the flip chip todetect the defect in the attachment between the flip chip and thesubstrate.
 2. The device of claim 1, further comprising a translationstage for x-y raster scanning the flip chip top surface relative to thegeneration and detection laser spots.
 3. The device of claim 1, whereinthe attachment comprises an underfill between the bottom of the flipchip and the substrate.
 4. The device of claim 1, wherein the attachmentcomprises an area array of solder balls between contact pads on the flipchip and contact pads on the substrate.
 5. The device of claim 1,wherein the defect is a void.
 6. The device of claim 1, wherein thedefect is a solder ball of insufficient mass.
 7. The device of claim 1,wherein the flip chip comprises a semiconductor material selected fromthe group consisting of silicon, germanium, gallium arsenide, indiumphosphide, and silicon carbide.
 8. The device of claim 1, wherein thewavelength of the generation laser beam is predetermined such that thephoton energy of the generation laser light exceeds the bandgap energyof the semiconductor material comprising the flip chip.
 9. The device ofclaim 1, wherein the wavelength of the generation laser beam ispredetermined such that the generation laser light is absorbed in thebulk of the semiconductor material comprising the flip chip.
 10. Thedevice of claim 1, wherein the wavelength of the detection laser beam ispredetermined such that the photon energy of the detection laser lightexceeds the bandgap energy of the semiconductor material comprising theflip chip.
 11. The device of claim 1, wherein the flip chip comprisessilicon and the wavelengths of the generation laser beam and thedetection laser beam are both 532 nm.
 12. The device of claim 1, whereinthe flip chip comprises silicon and the wavelength of the generationlaser beam is 532 nm and the wavelength of the detection laser beam is515 nm.
 13. The device of claim 1, wherein the flip chip comprisessilicon and the wavelength of the generation laser beam is 1064 nm andthe wavelength of the detection laser beam is 532 nm.
 14. The device ofclaim 1, wherein the substrate comprises a ceramic material.
 15. Thedevice of claim 1, wherein the substrate comprises a polymer-basedmaterial.
 16. The device of claim 1, wherein the diameter of thegeneration laser spot is 100 μm or less.
 17. The device of claim 1,wherein the diameter of the detection laser spot is 100 μm or less. 18.The device of claim 1, wherein the distance between the center of thegeneration laser spot and the center of the detection laser spot is lessthan 300 μm.
 19. The device of claim 1, wherein the top surface of theflip chip is x-y raster scanned with respect to the generation anddetection laser beam spots.
 20. The device of claim 1, wherein the timeframe for the compared waveforms is limited to the time required for thefirst longitudinal wave reflection to reach the detection spot.